The present invention relates to electrical current measuring devices, and more particularly, to an improved method and apparatus for measuring DC current flowing through a wire without making any direct connection to the wire.
For purposes of the discussion herein, reference will be made to two classes of magnetic material. One class is that of high permeability, high mu, soft magnetic material whose magnetic domains can rotate or turn easily with little or no friction. The other class is that of high retentivity, permanent magnet, hard magnetic material whose magnetic domains turn or rotate with difficulty because of significant friction. These two classes of magnetic material are mutually exclusive since a material cannot have both types of magnetic properties at the same time.
If a torroidal core of soft magnetic material is placed around a current carrying wire, the magnetomotive force created by the current in the wire will orient the domains in the core so that North and South poles are aligned with the central, longitudinal axis of the core. A net increase in magnetic flux results, and the core is said to be magnetized.
If a core of soft magnetic material having a split ring construction is removed around a current carrying wire, the core will lose its magnetism because of self-demagnetization. Thus, there is no retained magnetism in the split ring core which can be measured as an indicator of the amount of current flowing through the wire.
U.S. Pat. No. 2,996,682, Miller, discloses in FIG. 11 a current measuring device in which a closed core of what is apparently soft magnetic material contains a central, broken ring conductor. Three spaced apart coils are wound circumferentially about the core. A continuous current may be applied to the central conductor and a DC current to be measured may be applied to one of the circumferential coils. The current induced in one of the other circumferential coils is detected to provide an indication of the DC current being measured.
If a toroidal core of a hard magnetic material is placed around a current carrying wire the magnetomotive force created by the current will not rotate or otherwise change the direction of the domains until the friction of turning the domains is overcome. Thus, in some instances the current in the wire does not change the magnetization of the core and there is no increase or decrease in the flux until the current reaches a predetermined value. However, if the core of hard magnetic material has a split ring construction, it can then be removed from the wire without a loss in magnetization because the friction characteristic of this type of magnetic material prevents the domains from reorienting themselves and demagnetizing the ring. Thus, the information in the magnetic material due to magnetization thereof is not lost when the split ring is removed from about the wire as happens in the case of soft magnetic core.
U.S. Pat. No. 1,942,065, Lorraine, discloses a surge current measuring device. A pair of cylindrical elements made of hard magnetic material are placed in the vicinity of a current carrying wire. As shown in FIG. 7 of the Lorraine patent, the magnetic elements may be subjected to a continuous steady state DC current of known amplitude through coils circumferentially wound about the elements. The resultant magnetic flux induced in the elements after a surge of current through the wire is measured by placing the elements in an instrument to provide a quantative indication of the current surge. In the Lorraine patent, the final amount of magnetic flux induced in the elements depends upon their prior history of magnetization. The magnetic flux induced in the elements as a result of a current surge in the adjacent wire is generally non-linear because the method depends upon the non-linear hysteresis curves of the hard magnetic material of the elements. The time at which the magnetic flux is induced in the elements is unknown since it results from a random current surge. The current surge may be due to an event such as a flash over in a power transmission line during a lightening discharge. Furthermore the Lorraine method is primarily adapted for measuring the maximum value of a surge current whether it is unidirectional or oscillatory. It is not adapted for measuring a constant DC current flowing through a wire.
U.S. Pat. No. 4,005,380 Heilmann et al, discloses a clamp-on device for detecting current pulses in a wire. Ferrite core halves are mounted in respective jaw shells of a spring biased clamp. U.S. Pat. No. 3,706,032, Vikstrom, discloses another clamp-on DC current detector.
Other patents of interest in this field are U.S. Pat. Nos. 1,825,514; 2,266,624; 2,760,158; 3,323,056; 3,333,192; 3,396,338; 3,426,276; 3,584,299; 3,781,682; 3,812,428; 3,882,387; 3,913,015; and 3,984,798.